1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus for reproducing information recorded on an optical disc, and particularly relates to processing for correcting deviation of light received by a photodetector caused by lens shifting at the time of search operation by an optical pick-up.
2. Description of Related Arts
Conventionally, with an optical disc reproducing apparatus, a tracking servo for an optical pick-up is carried out in accordance with a tracking error signal. More specifically, the tracking servo for the optical pick-up is carried out in accordance with a track cross signal where pulse-width changes according to an amount of shifting (amount of deviation toward an inner periphery-side or outer periphery-side of a track on the optical disc) of an objective lens provided in the optical pick-up relative to a sled on which the optical pick-up is mounted.
For example, when the objective lens is shifted to the inner periphery side of a track on the optical disc, as a result of a tracking servo based on a track cross signal at this time, in order to return the objective lens to a normal position with respect to the track, tracking control is performed. In this tracking control, a tracking drive voltage is applied to a tracking actuator so that the objective lens is moved in a direction towards the outer periphery side with the objective lens then adopting a normal position with respect to the track.
However, with conventional optical disc reproducing apparatus, the processing described in the following is carried out by the system controller until the focus servo goes on. Here, a description is given where a DVD is loaded into the optical disc reproducing apparatus.
The system controller then turns a DVD laser diode on. A sum signal (A+B+C+D) and a focus error signal (A+C)−(B+D) are then acquired from a four segment photodetector. Next, the DVD laser diode is turned off, and a CD laser diode is turned on. Then, as described above, the sum signal (A+B+C+D) and the focus error signal (A+C)−(B+D) are acquired.
After this, when the system controller determines that the inserted disc is a DVD as a result of the level of the sum signal (A+B+C+D) obtained with the DVD laser diode turned on being larger than the level of the sum signal (A+B+C+D) obtained with the CD laser diode turned on, the levels of detection signals A, B, C, and D from the four segment photodetector with the DVD laser diode turned on are individually adjusted so as to all be the same level. Next, the system controller carries out S-shaped signal balance adjustment of the acquired focus error signal and focus pulling-in setting for pulling in the focus servo with the DVD laser diode turned on. The focus servo is then turned on.
With optical disc reproducing apparatus of the related art, after turning the focus servo on and achieving a focus-on state using the processing described above, the tracking servo is turned on, and a spot of laser light from the optical pick-up is tracked on to a track on the optical disc. However, in processing for up to the focus servo being turned on as described above, when detracking (a state where the track-on is achieved at a location deviating from the center of the track) occurs, laser light is irradiated on a location deviating from the center of the track even when a focus-on state is achieved with the state remaining as is. As a result, jitter of the reproduced signal worsens and the quality of reproduction from the optical disc falls.
In the technology of the related art disclosed in JP-A-H09-326123, at the time of start-up or re-starting of the apparatus, when a convergent lens is moved away after coming close to the disc, or when a convergent lens comes close to the disc after being moved away, an amplitude of an S-shaped signal appearing on the focus error signal each time a convergent point of the optical beam passes through each information surface is measured. The focus system gain is then switched over so as to give a predetermined amplitude, and an optimum pulling-in level is set. However, in the technology of the related art, countermeasures for the case where detracking occurs are not disclosed for the processing up to the focus servo going on, and the problem described above therefore cannot be solved.
In the technology of the related art disclosed in JP-A-H04-252435, (A+D)−(B+C) is calculated and main information containing a tracking error component and an offset error component accompanying shift of an objective lens is created. Next, (A+B)−(C+D) is calculated, a transform coefficient is applied to the result so as to give a transformation to an offset error component. Correction information is then created, and tracking control is carried out. However, in this technology of the related art, countermeasures for the case where detracking occurs are not disclosed for the processing up to the focus servo going on. The problems described above therefore cannot be solved. Further, when optical discs that are subject to reproduction are, for example, DVD-ROMs and DVD-RWs, reflectance of the DVD-ROMs is greater than reflectance of the DVD-RWs. This means that the result for (A+D)−(B+C) of the DVD-ROMs is large compared to that of the DVD-RWs. When a number of types of optical disc are handled, the results of calculating (A+D)−(B+C) become different and become therefore ambiguous. This means that the precision of tracking control is effectively lowered.
In the related technology disclosed in JP-A-2005-203062, an amplitude of a phase difference tracking error signal is measured in the vicinity of an end section of an inner periphery-side and in the vicinity of an end section of an outer periphery-side within a recording region of an optical disc and minimum values are extracted. Both of the extracted minimum values are then compared, and the signal of the smaller amplitude is selected. An amplification rate for a phase difference tracking error signal is then decided in such a manner that it is always possible to detect the phase difference tracking error signal based on this smaller signal and a value set in advance. However, in this technology of the related art also, countermeasures for the case where detracking occurs are not disclosed for the processing up to the focus servo going on and the problem described above therefore cannot be solved.